1. Field of the Invention
The present invention relates to an electroluminescent (EL) device for use in instruments as a segment or a matrix display device of an emissive type, in displays and the like of various types of information terminals, etc.
2. Related Arts
Electroluminescent devices known heretofore comprise a luminescent layer based on a compound of a Group II element of periodic table with a Group VI element (referred to simply hereinafter as "a Group II-VI compound") such as zinc sulfide (ZnS) or strontium sulfide (SrS) doped with an element which functions as a luminescent center. Those devices are based on the luminescent phenomenon which occurs when an electric field is applied to the luminescent layer, and are believed promising as components of a flat panel display of an emissive type. FIG. 7 shows a schematic cross-sectional view of a generally utilized EL device 10. The EL device 10 comprises a glass substrate 1 as an insulating substrate, having thereon layers formed sequentially in the order of: a first electrode 2 made of an optically transparent ITO (indium tin oxide) film and the like; a first insulating layer 3 made of tantalum pentaoxide (Ta.sub.2 O.sub.5) and the like; a luminescent layer 4; a second insulating layer; and a second electrode 6. The ITO film is a transparent conductive film based on indium oxide (In.sub.2 O.sub.3) doped with tin (Sn), and is widely utilized as a transparent electrode.
The luminescent layer 4 may be a zinc sulfide (ZnS) layer doped with an element such as manganese (Mn), terbium (Tb), or samarium (Sm) as a luminescent center, or a strontium sulfide (SrS) layer doped with cerium (Ce) which functions as the luminescent center.
The EL emission depends on the combination of the host material and the element that is added therein as the luminescent center. For instance, when manganese (Mn) is added to a zinc sulfide (ZnS) host material, an amber emitting EL device can be obtained. Accordingly, a green emission can be obtained by an EL device based on a ZnS layer doped with terbium (Tb), and a red emission can be obtained by an EL device based on the same host material but doped with samarium (Sm). A blue-green emitting EL device can be obtained from strontium sulfide (SrS) doped with cerium (Ce).
In general, as a SrS:Ce (SrS doped with Ce; hereinafter the same) based EL device emits a blue-green light, a filter is necessary to use it as a blue-emitting device. However, a high brightness is necessary in case of using an SrS:Ce based EL device as a blue-emitting layer. By increasing the blue color purity of SrS:Ce based EL device and thereby using it filterless, a higher brightness can be obtained as compared with the case where a filter is used. Even if a filter should be used, the blue-emitting brightness can be ameliorated by increasing the blue color purity to thereby increase the filter transmittance.
The blue color purity of a SrS:Ce based EL device can be increased by reducing the doping concentration of Ce in SrS. According to a report (see Journal of Crystal Growth, 117 (1992) pp. 964-968), the blue color purity can be improved to yield CIE color indices x of 0.20 and y of 0.38 by controlling the concentration of doped Ce to 0.05 atomic percent. However, the reported case fails to obtain a high brightness emission with favorable blue purity; the brightness decreases with increasing blue color purity.
Another attempt to increase the blue color purity of a SrS:Ce based EL device comprises employing a stack of cerium-free SrS layers and SrS:Ce layers (see, for example, JP-A-Hei-2-236991; the term "JP-A-" as referred herein signifies "an unexamined published Japanese patent application"). However, this method requires complicated process steps. Moreover, the blue color purity as expressed by CIE coordinates decreases as to yield a value of x=0.20 and y=0.39 on applying a heat treatment for the improvement of brightness. It can be seen from the foregoing that an EL device improved in both brightness and blue color purity is yet to be developed.